1. Field of the Invention
The present invention relates to a curling device and an image forming apparatus.
2. Description of the Related Art
Heretofore, with an image forming apparatus such as a photocopier or printer which forms an image with an electrophotographic method, a toner image formed at an image forming unit is transferred to a sheet transported from a paper supply unit. Thereafter, the sheet is guided to a fusing device, and the unfused toner on the sheet is fused onto the sheet.
Incidentally, a fusing device of a heat pressure fusing method is known whereby a toner image is fused onto the sheet by applying heat and pressure simultaneously to a sheet. However, in the case of this fusing device, the sheet may be physically deformed (curled) in the event of fusing the toner image onto the sheet.
Now, such curling occurs due to the toner transferred onto the sheet melting by being heated at the fusing device, and by the toner shrinking in accordance with the cooling thereafter. Also, the amount of curling of the sheet is influenced by the amount of toner on the sheet and the difference in the amount of toner on the front and back sides of the sheet.
Recently, particularly with full-color image forming devices, the amount of toner on the paper has been increasing to correspond to various types of paper. Due to the ratio of color toner on the sheet increasing, the amount of curling increases accordingly. Therefore, in order to increase the quality of the output sheets, the curling of sheets needs to be corrected.
Thus, with a current image forming apparatus, a curling device is provided, which has a correcting unit for curling the sheet in the opposite direction as the curling occurring to the sheet, to correct the curling of the sheet.
Note that a correcting method with this correcting unit includes performing curling correction automatically based on the amount of toner on the front and back sides of the sheet when printing duplex, for example (see Japanese Patent Laid-Open No. 06-258906) or changing the curling correction amount automatically for multiplex copying and duplex copying (Japanese Patent Laid-Open No. 63-027372).
Also, correcting methods are known wherein the curling correction amount is automatically measured from the paper type or percentage of moisture content in the paper (see Japanese Patent Laid-Open No. 04-251067) or wherein the curling correction amount is automatically determined by sheet processing conditions or heat amount (see Japanese Patent Laid-Open No. 05-309971).
However, with such curling device and image forming apparatus, curling amounts from temperatures and moisture amounts differ based on recent increased paper product types with various paper properties, and thus automatically setting the optimal curling correction amount for a new sheet has become more difficult. Also, in order to improve quality of the output sheet which is the final product, a higher precision is required for the correction of curling amount after output, so as to avoid unsightly curling.
Thus, curling correction of the sheet has become difficult with automatic control only. As disclosed in Japanese Patent Laid-Open No. 2005-096892 and Japanese Patent Laid-Open No. 2002-080157, fine-tuning by user input is being considered, as opposed to automatic control, for curling correction.
Japanese Patent Laid-Open No. 2005-096892 discloses a device whereby manual change means for manually changing the curling correction amount is provided, the manually set correction value is stored, and the stored correction amount can be reused. Also, the setting conditions used with automatic control means can be replaced with the manually set correction value. The configuration of Japanese Patent Laid-Open No. 2005-096892 applies curling in one direction and cannot respond to curling in the opposite direction.
With the configuration described in Japanese Patent Laid-Open No. 2002-080157, a sheet is transported to one of a first curling portion which adds curling in one direction and a second curling portion which adds curling in the other direction, thereby responding to curling in both directions. The levels of curling correction force by the two curling portions can each be set.
Now, for example, the user himself/herself must think and set the level of curling correction force so as to minimize curling as to the opposite direction of the sheet having passed through the curling portion, thus operability is poor. Particularly, one case that can be considered is a situation wherein the sheet having passed through the first curling portion is curled, for example, and the second curling portion is used, a sheet with a straight shape is obtained. In this case, with the configuration in Japanese Patent Laid-Open No. 2002-080157, the user himself must switch the curling portion to operate, and perform setting of the curling correction force levels himself also. Accordingly, depending on the user, the user may not think to perform such switching settings, or may erroneously set the curling correcting force levels according to the switching settings. Thus, the configuration in Japanese Patent Laid-Open No. 2002-080157 does not have high operability for a user when performing curling fine-tuning input.